Positive electrode plate of lithium ion secondary battery, lithium ion secondary battery, and method of producing positive electrode plate of lithium ion secondary battery

ABSTRACT

A positive electrode plate of a lithium ion secondary battery includes a current collector foil, an active material layer including positive electrode active material particles containing lithium oxide on the current collector foil, and a protective conductive layer that does not include the positive electrode active material particles and includes a conductive material and a binding agent on the active material layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2018-001199 filed on Jan. 9, 2018, which is incorporated herein byreference in its entirety including the specification, drawings andabstract.

BACKGROUND 1. Technical Field

The present disclosure relates to a positive electrode plate of alithium ion secondary battery in which an active material layer isprovided on a current collector foil, a lithium ion secondary batteryincluding the positive electrode plate, and a method of producing apositive electrode plate.

2. Description of Related Art

As a positive electrode plate used for a lithium ion secondary battery(hereinafter simply referred to as a “battery”), a positive electrodeplate in which an active material layer containing positive electrodeactive material particles made of lithium oxide is formed on a currentcollector foil is known. In addition, as positive electrode activematerial particles made of lithium oxide, lithium nickel cobalt aluminumcomposite oxide particles, lithium nickel cobalt manganese compositeoxide particles, olivine type iron phosphate lithium particles, spineltype lithium manganese oxide particles, and the like are known. Forexample, in Japanese Unexamined Patent Application Publication No.2016-88776 (JP 2016-88776 A), as positive electrode active materialparticles, lithium nickel cobalt aluminum composite oxide particles aredisclosed (refer to the scope of claims of JP 2016-88776 A).

SUMMARY

However, when positive electrode active material particles made oflithium oxide are in contact with moisture in the atmosphere, surfacesof the particles react with water (H₂O) and lithium hydroxide (LiOH) isgenerated (Li₂O+H₂O→2LiOH). Further, the lithium hydroxide reacts withcarbon dioxide (CO₂) in the atmosphere and lithium carbonate (Li₂CO₃) isgenerated (2LiOH+CO₂→Li₂CO₃+H₂O). Lithium carbonate generated onparticle surfaces of positive electrode active material particles is aresistor. In addition, when positive electrode active material particlesreact with water and lithium ions are released from positive electrodeactive material particles, a crystal structure of positive electrodeactive material particles changes and insertion and removal of lithiumions in positive electrode active material particles become difficult.Therefore, in a battery using the positive electrode plate, an IVresistance becomes higher.

The present disclosure provides a positive electrode plate of a lithiumion secondary battery that can reduce an increase in an IV resistance ofa battery when the battery is formed due to contact with moisture andcarbon dioxide in the atmosphere, a lithium ion secondary battery usingthe positive electrode plate, and a method of producing a positiveelectrode plate of a lithium ion secondary battery.

A first aspect of the present disclosure is a positive electrode plateof a lithium ion secondary battery, including a current collector foil;an active material layer including positive electrode active materialparticles containing lithium oxide on the current collector foil; and aprotective conductive layer that does not include the positive electrodeactive material particles and includes a conductive material and abinding agent on the active material layer.

In the positive electrode plate of the lithium ion secondary battery,since the protective conductive layer is provided on the active materiallayer, moisture and carbon dioxide in the atmosphere are unlikely tocome in contact with positive electrode active material particles in theactive material layer when the positive electrode plate is handled.Therefore, it is possible to reduce generation of lithium hydroxide onparticle surfaces of the positive electrode active material particles inthe active material layer, and additionally, generation of lithiumcarbonate due to contact with moisture and carbon dioxide, and change ina crystal structure on particle surfaces. Therefore, in the batteryusing the positive electrode plate, compared to a battery using apositive electrode plate having no protective conductive layer on anactive material layer, a positive electrode plate in which an IVresistance of the battery is reduced can be obtained. Moreover, sincethe conductive material is included in the protective conductive layer,compared to a positive electrode plate in which the conductive materialis not included in the protective conductive layer, the conductivity ofthe positive electrode plate in the thickness direction can be improved.

In the first aspect, the protective conductive layer may include amoisture absorbent.

In the positive electrode plate, since the protective conductive layerincludes a moisture absorbent, even if the positive electrode platecomes in contact with moisture in the atmosphere, the moisture isabsorbed by the moisture absorbent included in the protective conductivelayer. Therefore, it is possible to reduce the amount of moisture thatreaches the active material layer below the protective conductive layer.Therefore, it is possible to effectively reduce generation of lithiumhydroxide on particle surfaces, and additionally, generation of lithiumcarbonate due to moisture in contact with positive electrode activematerial particles in the active material layer, and change in a crystalstructure on particle surfaces. Therefore, compared to a battery using apositive electrode plate in which no moisture absorbent is included in aprotective conductive layer, a positive electrode plate in which an IVresistance of the battery is further reduced can be obtained.

Here, examples of the “moisture absorbent” include, for example, silicagel, gypsum, zeolite such as Molecular Sieve (registered trademark)(MS), and aluminum oxide, boehmite, oxidized calcium, calcium chloride,and diphosphorus pentoxide powders.

In the first aspect, the moisture absorbent may be a chemical moistureabsorbent that adsorbs water through a chemical reaction.

In the positive electrode plate, when the moisture absorbent is achemical moisture absorbent, it adsorbs moisture more easily than with aphysical moisture absorbent (for example, zeolite, aluminum oxide, andboehmite powders) that adsorbs water physically. Therefore, it ispossible to effectively reduce the amount of moisture that reaches theactive material layer below the protective conductive layer when thepositive electrode plate comes in contact with moisture in theatmosphere. Therefore, a positive electrode plate in which an IVresistance of the battery is more effectively reduced can be obtained.

Here, examples of the “chemical moisture absorbent” include silica gel,gypsum, oxidized calcium, calcium chloride, and diphosphorus pentoxidepowders. Since gypsum is inexpensive and easily handled, it is used as achemical moisture absorbent in some embodiments. Here, silica gel notonly adsorbs water physically but also adsorbs water chemically usingsilanol groups. Thus, in the present disclosure, the chemical moistureabsorbent described above is included.

In the first aspect, the moisture absorbent may be an anhydrite powder.

In the first aspect, the positive electrode active material particlesincluded in the active material layer may have a property in which a pHof a liquid dispersion in which 1 g of the positive electrode activematerial particles is dispersed in 49 g of water is 11.3 or more.

In the positive electrode plate, as the positive electrode activematerial particles made of lithium oxide, positive electrode activematerial particles having a property in which a pH of a liquiddispersion is 11.3 or more is used. Such positive electrode activematerial particles react with particularly water and carbon dioxide,easily generate lithium hydroxide, and additionally, lithium carbonate,and an IV resistance is likely to be higher in a battery using thepositive electrode plate. In some embodiments, the protective conductivelayer is provided on the active material layer and moisture and carbondioxide in the atmosphere do not come in contact with the positiveelectrode active material particles.

In the first aspect, a layer thickness t2 of the protective conductivelayer may be thinner than a layer thickness t1 of the active materiallayer (t2<t1).

In the first aspect, the layer thickness of the protective conductivelayer may be 2 μm or more.

In the positive electrode plate, since the layer thickness t2 of theprotective conductive layer is thinner than the layer thickness t1 ofthe active material layer, compared to when the layer thickness t2 ofthe protective conductive layer is thicker than the layer thickness t1of the active material layer, it is possible to reduce a decrease in abattery capacity (battery capacity per unit thickness of the positiveelectrode plate) according to the provision of the protective conductivelayer.

However, when the layer thickness t2 of the protective conductive layeris too thin, the active material layer is easily partially exposed. Whenthe active material layer is partially exposed, moisture and carbondioxide in the atmosphere easily come in contact with the positiveelectrode active material particles in the active material layer. Insome embodiments, the layer thickness t2 of the protective conductivelayer is 2 μm or more.

A second aspect of the present disclosure is a lithium ion secondarybattery, including the positive electrode plate according to the firstaspect and a negative electrode plate.

In a positive electrode plate using the lithium ion secondary battery, aprotective conductive layer is provided on an active material layer.Therefore, in the battery, compared to a battery using a positiveelectrode plate having no protective conductive layer on an activematerial layer, it is possible to reduce an IV resistance of the batteryas described above.

A third aspect of the present disclosure is a method of producing apositive electrode plate of a lithium ion secondary battery, includingforming an undried active material layer including positive electrodeactive material particles containing lithium oxide on a currentcollector foil (first undried layer forming process), forming an undriedprotective conductive layer that does not include positive electrodeactive material particles and includes a conductive material and abinding agent on the undried active material layer (second undried layerforming process), and drying the undried active material layer and theundried protective conductive layer simultaneously and forming theactive material layer and the protective conductive layer (simultaneousdrying process).

As a method of producing a positive electrode plate, for example, amethod in which an undried active material layer is formed and thendried to form an active material layer, and then an undried protectiveconductive layer is formed on the active material layer and dried toform a protective conductive layer may be conceived. However, in thismethod, not only in a process of forming the active material layer andthen forming the undried protective conductive layer thereon, but alsoin a process of drying the undried active material layer with hot airand forming the active material layer, positive electrode activematerial particles come in contact with moisture and carbon dioxide inthe atmosphere. Therefore, generation of lithium hydroxide andadditionally lithium carbonate on particle surfaces of the positiveelectrode active material particles occur and change in a crystalstructure on particle surfaces may occur. As a result, an IV resistanceis higher in a battery using the positive electrode plate.

On the other hand, this method of producing a positive electrode plateincludes the first undried layer forming process, the second undriedlayer forming process and the simultaneous drying process. Before theundried active material layer is dried, since the undried protectiveconductive layer is formed on the undried active material layer, it ispossible to reduce contact of the positive electrode active materialparticles in the undried active material layer with moisture and carbondioxide in the atmosphere in the drying process. Therefore, it ispossible to reduce generation of lithium hydroxide and additionally,lithium carbonate on particle surfaces of the positive electrode activematerial particles, and change in a crystal structure on particlesurfaces. Therefore, in a battery using the positive electrode plate,compared to a battery using the positive electrode plate producedaccording to the above production method, it is possible to reduce an IVresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a perspective view of a battery according to Embodiments 1 and2;

FIG. 2 is a cross-sectional view of the battery according to Embodiments1 and 2;

FIG. 3 is a perspective view of a positive electrode plate according toEmbodiments 1 and 2;

FIG. 4 is a cross-sectional view of the positive electrode plateaccording to Embodiments 1 and 2;

FIG. 5 is a flowchart of a method of producing the battery according toEmbodiments 1 and 2;

FIG. 6 is a flowchart of a positive electrode plate producing processsubroutine according to Embodiments 1 and 2;

FIG. 7 is an explanatory diagram showing a method of producing thepositive electrode plate according to Embodiments 1 and 2; and

FIG. 8 is a graph showing IV resistance ratios of batteries according toExamples 1 and 2 and a comparative example.

DETAILED DESCRIPTION OF EMBODIMENTS Embodiment 1

A first embodiment of the present disclosure will be described belowwith reference to the drawings. FIG. 1 and FIG. 2 show a perspectiveview and a cross-sectional view of a lithium ion secondary battery(hereinafter simply referred to as a “battery”) 1 according toEmbodiment 1. In addition, FIG. 3 and FIG. 4 show a perspective view anda cross-sectional view of a positive electrode plate 31 of the battery1. Here, in the following description, a battery longitudinal directionBH, a battery transverse direction CH and a battery thickness directionDH in the battery 1 will be defined as the directions shown in FIG. 1and FIG. 2. In addition, in the following description, a longitudinaldirection EH, a width direction FH and a thickness direction GH in thepositive electrode plate 31 will be defined as the directions shown inFIG. 3 and FIG. 4.

The battery 1 is a rectangular and closed type lithium ion secondarybattery mounted on a vehicle such as a hybrid vehicle and a plug-inhybrid vehicle, and an electric vehicle. The battery 1 includes abattery case 10, an electrode body 20 accommodated therein, a positiveelectrode terminal member 70 and a negative electrode terminal member 80supported by the battery case 10, and the like. In addition, anelectrolytic solution 17 is accommodated in the battery case 10 and apart thereof is impregnated in the electrode body 20. The electrolyticsolution 17 includes lithium hexafluorophosphate (LiPF₆) as a solute.

Of these, the battery case 10 has a rectangular parallelepiped box shapeand is made of a metal (in Embodiment 1, aluminum). The battery case 10includes a case main body member 11 having a bottomed rectangulartubular shape of which only an upper side is open and a rectangularplate-like case lid member 13 welded to allow closing of an opening ofthe case main body member 11. In the case lid member 13, the positiveelectrode terminal member 70 made of aluminum is fixed and insulatedfrom the case lid member 13. The positive electrode terminal member 70is connected to and conducts electricity with a positive electrodeexposed part 31 m of the positive electrode plate 31 of the electrodebody 20 in the battery case 10, and extends to the outside of thebattery through the case lid member 13. In addition, in the case lidmember 13, the negative electrode terminal member 80 made of copper isfixed and is insulated from the case lid member 13. The negativeelectrode terminal member 80 is connected to and conducts electricitywith a negative electrode exposed part 51 m of a negative electrodeplate 51 of the electrode body 20 in the battery case 10, and extends tothe outside of the battery through the case lid member 13.

The electrode body 20 has a flat shape and is accommodated in thebattery case 10 in a horizontal state. A bag-shaped insulation filmenclosure 19 made of an insulation film is disposed between theelectrode body 20 and the battery case 10. In the electrode body 20, thebelt-like positive electrode plate 31 and the belt-like negativeelectrode plate 51 are laminated with a pair of separators 61 and 61made of a belt-like resin porous member therebetween and wound in a flatshape around an axis.

The positive electrode plate 31 (refer to FIG. 3 and FIG. 4) has apositive electrode current collector foil 32 made of a belt-likealuminum foil. Within one main surface 32 a of the positive electrodecurrent collector foil 32, on an area which is a part of the positiveelectrode plate 31 in the width direction FH and extends in thelongitudinal direction EH, an active material layer 33 with a layerthickness t1=60 μm is formed in a belt shape. In addition, within theother main surface 32 b of the positive electrode current collector foil32, on an area which is a part of the positive electrode plate 31 in thewidth direction FH and extends in the longitudinal direction EH,similarly, an active material layer 34 with a layer thickness t1=60 μmis formed in a belt shape.

These active material layers 33 and 34 include positive electrode activematerial particles 41 made of lithium oxide, a conductive material 42, abinding agent 43 and a dispersant 44. In Embodiment 1, as the positiveelectrode active material particles 41 made of lithium oxide, lithiumnickel cobalt aluminum composite oxide particles having a layered rocksalt structure, specifically, Li_(1.02) (Ni_(08.2)Co_(0.14)Al_(0.04))O₂particles having an average particle size of 11 min are used. Thepositive electrode active material particles 41 have a property in whicha pH of a liquid dispersion in which 1 g of the positive electrodeactive material particles 41 is dispersed in 49 g of water becomespH=11.3 or more (in Embodiment 1, pH=11.6). In addition, in Embodiment1, acetylene black (AB) is used as the conductive material 42,polyvinylidene fluoride (PVDF) is used as the binding agent 43, and ananionic dispersant is used as the dispersant 44. A formulation ratio byweight between the positive electrode active material particles 41, theconductive material 42, the binding agent 43, and the dispersant 44 is92:6:1:1.

In addition, on one active material layer 33, over its entire surface, aprotective conductive layer 35 having a layer thickness t2 that isthinner than the layer thickness t1=60 μm of the active material layer33 (t2<t1) and having a layer thickness of 2 μm or more (t2=6 μm) isformed. In addition, on the other active material layer 34, over itsentire surface, a protective conductive layer 36 having a layerthickness t2 that is thinner than the layer thickness t1=60 μm of theactive material layer 34 (t2<t1), and having a layer thickness of 2 μmor more (t2=6 μm) is formed. These protective conductive layers 35 and36 do not include positive electrode active material particles, andinclude a conductive material 45, a binding agent 46, a moistureabsorbent 47, and a dispersant 48.

In Embodiment 1, as the moisture absorbent 47, a chemical moistureabsorbent that adsorbs water according to a chemical reaction,specifically, an anhydrite powder, is used. In addition, AB is used asthe conductive material 45 like the conductive material 42 of the activematerial layers 33 and 34, PVDF is used as the binding agent 46 like thebinding agent 43 of the active material layers 33 and 34, and an anionicdispersant is used as the dispersant 48 like the dispersant 44 of theactive material layers 33 and 34. A formulation ratio by weight betweenthe conductive material 45, the binding agent 46, the moisture absorbent47, and the dispersant 48 is 30:6:64:1.

Here, one end of the positive electrode plate 31 in the width directionFH forms the positive electrode exposed part 31 m in which the positiveelectrode current collector foil 32 is exposed in the thicknessdirection GH without the active material layers 33 and 34 and theprotective conductive layers 35 and 36 in the thickness direction GH.The positive electrode terminal member 70 is welded to the positiveelectrode exposed part 31 m.

The negative electrode plate 51 has a belt-like negative electrodecurrent collector foil 52 made of copper foil. Within one main surfaceof the negative electrode current collector foil 52, on an area which isa part of the negative electrode plate 51 in the width direction andextends in the longitudinal direction, a negative electrode activematerial layer (not shown) is formed in a belt shape. In addition,within the other main surface of the negative electrode currentcollector foil 52, on an area which is a part of the negative electrodeplate 51 in the width direction and extends in the longitudinaldirection, a negative electrode active material layer (not shown) isformed in a belt shape. These negative electrode active material layersinclude negative electrode active material particles, a binding agentand a thickener. In Embodiment 1, graphite particles are used as thenegative electrode active material particles, styrene butadiene rubber(SBR) is used as the binding agent, and carboxymethyl cellulose (CMC) isused as the thickener.

As described above, in the positive electrode plate 31 of the battery 1,since the protective conductive layers 35 and 36 are provided on theactive material layers 33 and 34, moisture and carbon dioxide in theatmosphere are unlikely to come in contact with the positive electrodeactive material particles 41 in the active material layers 33 and 34when the positive electrode plate 31 is handled. Therefore, it ispossible to reduce generation of lithium hydroxide on particle surfacesof the positive electrode active material particles 41 in the activematerial layers 33 and 34, and additionally, generation of lithiumcarbonate due to contact with moisture and carbon dioxide and change ina crystal structure on particle surfaces. Therefore, in the battery 1using the positive electrode plate 31, compared to a battery using apositive electrode plate having no protective conductive layers 35 and36 on the active material layers 33 and 34, the positive electrode plate31 in which an IV resistance of the battery 1 is reduced can beobtained. Moreover, since the conductive material 45 is included in theprotective conductive layers 35 and 36, compared to a positive electrodeplate in which the conductive material 45 is not included in theprotective conductive layers 35 and 36, the conductivity of the positiveelectrode plate 31 in the thickness direction GH can be improved.

In addition, in the positive electrode plate 31 of Embodiment 1, sincethe protective conductive layers 35 and 36 include the moistureabsorbent 47, even if the positive electrode plate 31 comes in contactwith moisture in the atmosphere, the moisture is absorbed by themoisture absorbent 47 included in the protective conductive layers 35and 36. Therefore, it is possible to reduce the amount of moisture thatreaches the active material layers 33 and 34 below the protectiveconductive layers 35 and 36. Therefore, it is possible to effectivelyreduce generation of lithium hydroxide on particle surfaces, andadditionally, generation of lithium carbonate, due to moisture incontact with the positive electrode active material particles 41 in theactive material layers 33 and 34, and change in a crystal structure onparticle surfaces. Therefore, compared to a battery using a positiveelectrode plate in which the moisture absorbent 47 is not included inthe protective conductive layers 35 and 36, the positive electrode plate31 in which an IV resistance of the battery 1 is further reduced can beobtained. In particular, in this embodiment, since the moistureabsorbent 47 is a chemical moisture absorbent (in Embodiment 1, gypsum),moisture is more easily adsorbed than with a physical moistureabsorbent. Therefore, the positive electrode plate 31 in which an IVresistance of the battery 1 is more effectively reduced can be obtained.

In addition, in the positive electrode plate 31 of Embodiment 1, as thepositive electrode active material particles 41 made of lithium oxide,positive electrode active material particles having a property in whichthe liquid dispersion has a pH of 11.3 or more are used. Such positiveelectrode active material particles 41 react with particularly water andcarbon dioxide, easily generate lithium hydroxide, and additionally,lithium carbonate, and an IV resistance is likely to be higher in thebattery 1 using the positive electrode plate 31. In some embodiments,the protective conductive layers 35 and 36 are provided on the activematerial layers 33 and 34 and moisture and carbon dioxide in theatmosphere do not come in contact with the positive electrode activematerial particles 41.

In addition, in the positive electrode plate 31 of Embodiment 1, sincethe layer thickness t2 of the protective conductive layers 35 and 36 isthinner than the layer thickness t1 of the active material layers 33 and34 (t2<t1), compared to when the layer thickness t2 of the protectiveconductive layers 35 and 36 is thicker than the layer thickness t1 ofthe active material layers 33 and 34, it is possible to reduce adecrease in a battery capacity (battery capacity per unit thickness ofthe positive electrode plate 31) according to the provision of theprotective conductive layers 35 and 36. On the other hand, since thelayer thickness t2 of the protective conductive layers 35 and 36 is setto 2 μm or more (in Embodiment 1, t2=6 μm), it is possible to preventthe active material layers 33 and 34 from being partially exposed (theentire surfaces of the active material layers 33 and 34 can be coveredwith the protective conductive layers 35 and 36).

In addition, in the positive electrode plate 31 used for the battery 1of Embodiment 1, the protective conductive layers 35 and 36 are providedon the active material layers 33 and 34. Therefore, in the battery 1,compared to a battery using a positive electrode plate having noprotective conductive layers 35 and 36 on the active material layers 33and 34, it is possible to reduce an IV resistance of the battery 1 asdescribed above.

Next, a method of producing the battery 1 including a method ofproducing the positive electrode plate 31 will be described (refer toFIG. 5 to FIG. 7). Here, in Embodiment 1, processes of a “positiveelectrode plate producing process S1” and a “negative electrode plateproducing process S2” to a “battery assembling process S4” are performedunder environments of 25° C., a humidity of 60%, and a dew point (DP)temperature of 16° C. First, the “positive electrode plate producingprocess S1” is performed to produce the positive electrode plate 31. Afirst paste DP1 used for forming the active material layers 33 and 34and a second paste DP2 used for forming the protective conductive layers35 and 36 are prepared in advance.

Specifically, the positive electrode active material particles 41 madeof lithium oxide (in Embodiment 1, lithium nickel cobalt aluminumcomposite oxide), the conductive material 42 (in Embodiment 1, AB), thebinding agent 43 (in Embodiment 1, PVDF) and the dispersant 44 (inEmbodiment 1, an anionic dispersant) are kneaded together with adispersion medium (in Embodiment 1, N-methyl-2-pyrrolidone (NMP)), andthereby the first paste DP1 is obtained. A formulation ratio by weightbetween the positive electrode active material particles 41, theconductive material 42, the binding agent 43, and the dispersant 44 is92:6:1:1. In addition, a solid fraction NV of the first paste DP1 is setto 70 wt % (a proportion of NMP is 30 wt %).

In addition, the conductive material 45 (in Embodiment 1, AB), thebinding agent 46 (in Embodiment 1, PVDF), the moisture absorbent 47 (inEmbodiment 1, anhydrite powder) and the dispersant 48 (in Embodiment 1,an anionic dispersant) are kneaded together with a dispersion medium (inEmbodiment 1, NMP), and thereby the second paste DP2 is obtained. Aformulation ratio by weight between the conductive material 45, thebinding agent 46, the moisture absorbent 47, and the dispersant 48 is30:6:64:1. In addition, a solid fraction NV of the second paste DP2 isset to 25 wt % (a proportion of NMP is 75 wt %).

Then, among subroutines of the positive electrode plate producingprocess S1 shown in FIG. 6, in “one side first undried layer formingprocess S11,” using a die coating device 100 (refer to FIG. 7), thefirst paste DP1 is applied to one main surface 32 a of the positiveelectrode current collector foil 32 and an undried active material layer33 x is formed. The die coating device 100 includes a coating die 110for applying the first paste DP1 to the positive electrode currentcollector foil 32, a backup roller 120 for transporting the positiveelectrode current collector foil 32, a pump (not shown) for deliveringthe first paste DP1 to the coating die 110, and the like. In the oneside first undried layer forming process S11, a predetermined dischargeamount of the first paste DP1 is discharged from the coating die 110toward the main surface 32 a of the positive electrode current collectorfoil 32 that is transported by the backup roller 120, and a belt-likecoating film (the undried active material layer 33 x) is continuouslyformed on the main surface 32 a of the positive electrode currentcollector foil 32.

Subsequently, in “one side second undried layer forming process S12,”before the undried active material layer 33 x is heated and dried, thesecond paste DP2 is applied to the undried active material layer 33 xusing a spray coating device 200 (refer to FIG. 7), and an undriedprotective conductive layer 35 x is formed. The spray coating device 200includes a spray gun 210 for spraying and applying the second paste DP2to the undried active material layer 33 x, a pump (not shown) fordelivering the second paste DP2 to the spray gun 210, and the like. Inthe one side second undried layer forming process S12, a predeterminedspray amount of the second paste DP2 is sprayed from the spray gun 210toward the undried active material layer 33 x of the positive electrodecurrent collector foil 32 in which the undried active material layer 33x is formed and which is transported by transport rollers 250 and 260,and a coating film (the undried protective conductive layer 35 x) iscontinuously formed on the entire surface of the undried active materiallayer 33 x.

Subsequently, in “one side simultaneous drying process S13,” using adrying device 300 (refer to FIG. 7), the undried active material layer33 x and the undried protective conductive layer 35 x on the mainsurface 32 a of the positive electrode current collector foil 32 aresimultaneously dried, and the active material layer 33 and theprotective conductive layer 35 are formed. Specifically, the positiveelectrode current collector foil 32 in which the undried active materiallayer 33 x and the undried protective conductive layer 35 x are formedis transported into the drying device 300, hot air is blown to theundried protective conductive layer 35 x, the undried protectiveconductive layer 35 x and the undried active material layer 33 xtherebelow are heated and dried, and thus the protective conductivelayer 35 and the active material layer 33 are formed. Therefore, asingle-sided positive electrode plate 31 y including the active materiallayer 33 and the protective conductive layer 35 on the main surface 32 aof the positive electrode current collector foil 32 is formed.

Next, in “the other side first undried layer forming process S14,” thefirst paste DP1 is applied to the other main surface 32 b of thepositive electrode current collector foil 32, and an undried activematerial layer 34 x is formed. The other side first undried layerforming process S14 is performed using the die coating device 100 in thesame manner as in the one side first undried layer forming process S11.Subsequently, in “the other side second undried layer forming processS15,” before the undried active material layer 34 x is heated and dried,the second paste DP2 is applied to the undried active material layer 34x, and an undried protective conductive layer 36 x is formed. The otherside second undried layer forming process S15 is performed using thespray coating device 200 in the same manner as in the one side secondundried layer forming process S12. Subsequently, in “the other sidesimultaneous drying process S16,” the undried active material layer 34 xand the undried protective conductive layer 36 x on the main surface 32b of the positive electrode current collector foil 32 are simultaneouslydried, and the active material layer 34 and the protective conductivelayer 36 are formed. The other side simultaneous drying process S16 isperformed using the drying device 300 in the same manner as in the oneside simultaneous drying process S13. Therefore, a positive electrodeplate 31 z including the active material layers 33 and 34 and theprotective conductive layers 35 and 36 on both main surfaces 32 a and 32b of the positive electrode current collector foil 32 is formed.

Next, in a “pressing process S17,” the positive electrode plate 31 z ispressed by a roll press machine (not shown), and the density of theactive material layers 33 and 34 and the protective conductive layers 35and 36 increases. Thus, the positive electrode plate 31 is produced.

In addition, separately, in the “negative electrode plate producingprocess S2,” the negative electrode plate 51 is produced. Negativeelectrode active material particles (in Embodiment 1, graphiteparticles), a binding agent (in Embodiment 1, SBR) and a thickener (inEmbodiment 1, CMC) are kneaded together with a dispersion medium (inEmbodiment 1, water) in advance, and a negative electrode paste isprepared. Then, the negative electrode paste is applied to one mainsurface of the negative electrode current collector foil 52 by diecoating, and an undried negative electrode active material layer (notshown) is formed, and then heated and dried to form a negative electrodeactive material layer (not shown). Similarly, a negative electrode pasteis also applied to the other main surface of the negative electrodecurrent collector foil 52, and an undried negative electrode activematerial layer (not shown) is formed and then heated and dried to form anegative electrode active material layer (not shown). Then, the negativeelectrode plate is pressed and the density of the negative electrodeactive material layer increases. Thus, the negative electrode plate 51is produced.

Next, in the “electrode body forming process S3,” the electrode body 20is formed. Specifically, the belt-like positive electrode plate 31 andthe belt-like negative electrode plate 51 are laminated with twobelt-like separators 61 and 61 therebetween and wound around an axisusing a winding core. Further, this is compressed into a flat shape anda flat wound type electrode body 20 is formed (refer to FIG. 2).

Next, in the “battery assembling process S4,” the battery 1 isassembled. That is, the case lid member 13 is prepared, and the positiveelectrode terminal member 70 and the negative electrode terminal member80 are fixed thereto (refer to FIG. 1 and FIG. 2). Then, the positiveelectrode terminal member 70 and the negative electrode terminal member80 are welded to the positive electrode exposed part 31 m of thepositive electrode plate 31 and the negative electrode exposed part 51 mof the negative electrode plate 51 of the electrode body 20. Next, theelectrode body 20 is covered with the insulation film enclosure 19 andthis is inserted into the case main body member 11, and an opening ofthe case main body member 11 is closed with the case lid member 13.Then, the case main body member 11 and the case lid member 13 are weldedto form the battery case 10.

Next, in the “liquid injection process S5,” the electrolytic solution 17is injected into the battery case 10 from a liquid injection hole 13 h,and impregnated into the electrode body 20. Then, the liquid injectionhole 13 h is sealed with a sealing member 15. Here, the liquid injectionprocess S5 is performed under a dry environment of 25° C. and a dewpoint (DP) temperature of −30° C. or lower unlike processes of thepositive electrode plate producing process S1 to battery assemblingprocess S4, and then, in a “first charging process S6,” the battery 1 isinitially charged. The first charging process S6 is performed under anenvironment of 25° C., a humidity of 60%, and a dew point (DP)temperature of 16° C. like processes of the positive electrode plateproducing process S1 to the battery assembling process S4. Then, varioustests are performed on the battery 1. Thus, the battery 1 is completed.

As described above, the method of producing the positive electrode plate31 (the positive electrode plate producing process S1) includes the oneside first undried layer forming process S11, the one side secondundried layer forming process S12, and the one side simultaneous dryingprocess S13. Before the undried active material layer 33 x is dried, theundried protective conductive layer 35 x is formed on the undried activematerial layer 33 x. Therefore, in the drying process, it is possible toreduce contact of the positive electrode active material particles 41 inthe undried active material layer 33 x with moisture and carbon dioxidein the atmosphere. In addition, the method includes the other side firstundried layer forming process S14, the other side second undried layerforming process S15, and the other side simultaneous drying process S16.Before the undried active material layer 34 x is dried, the undriedprotective conductive layer 36 x is formed on the undried activematerial layer 34 x. Therefore, in the drying process, it is possible toreduce contact of the positive electrode active material particles 41 inthe undried active material layer 34 x with moisture and carbon dioxidein the atmosphere. Therefore, in the producing process of the positiveelectrode plate 31, it is possible to reduce generation of lithiumhydroxide on particle surfaces of the positive electrode active materialparticles 41, and additionally, generation of lithium carbonate, andchange in a crystal structure on particle surfaces. Therefore, in thebattery 1 using the positive electrode plate 31, compared to a batteryusing a positive electrode plate produced by a production method offorming the undried protective conductive layers 35 x and 36 x after theundried active material layers 33 x and 34 x are dried, it is possibleto reduce an IV resistance.

Embodiment 2

Next, a second embodiment will be described. In the positive electrodeplate 31 of the battery 1 of Embodiment 1, the moisture absorbent 47 isincluded in the protective conductive layers 35 and 36. On the otherhand, a positive electrode plate 531 of a battery 500 of Embodiment 2 isdifferent from that of Embodiment 1 in that no moisture absorbent isincluded in protective conductive layers 535 and 536.

That is, the positive electrode plate 531 of Embodiment 2 includes thesame positive electrode current collector foil 32 as in Embodiment 1,and the same active material layer 33 as in Embodiment 1 is formed onthe main surface 32 a and the same active material layer 34 as inEmbodiment 1 is formed on the other main surface 32 b. In addition, alsoin Embodiment 2, the protective conductive layer 535 is formed on oneactive material layer 33 and the protective conductive layer 536 isformed on the other active material layer 34. However, these protectiveconductive layers 535 and 536 do not include a moisture absorbent, andinclude the conductive material 45, the binding agent 46, and thedispersant 48. Here, like Embodiment 1, regarding the conductivematerial 45, the binding agent 46, and the dispersant 48, AB is used asthe conductive material 45, PVDF is used as the binding agent 46, and ananionic dispersant is used as the dispersant 48. In addition, aformulation ratio by weight between the conductive material 45, thebinding agent 46, and the dispersant 48 is 30:6:1.

Here, the battery 500 of Embodiment 2 is produced in the same manner asin the battery 1 of Embodiment 1. That is, the positive electrode plateproducing process S1 is performed, specifically, the one side firstundried layer forming process S11, the one side second undried layerforming process S12, the one side simultaneous drying process S13, theother side first undried layer forming process S14, the other sidesecond undried layer forming process S15, the other side simultaneousdrying process S16, and the pressing process S17 are sequentiallyperformed, and the positive electrode plate 531 is produced. However, inthe one side second undried layer forming process S12 and the other sidesecond undried layer forming process S15, regarding the second pasteDP2, using a paste (a paste including the conductive material 45, thebinding agent 46, and the dispersant 48) including no moisture absorbent47, undried protective conductive layers 535 x and 536 x are formed. Inaddition, like Embodiment 1, the negative electrode plate producingprocess S2, the electrode body forming process S3, the batteryassembling process S4, the liquid injection process S5 and the firstcharging process S6 are performed, and the battery 500 is produced.

In the positive electrode plate 531 of Embodiment 2, since theprotective conductive layers 535 and 536 are provided on the activematerial layers 33 and 34, moisture and carbon dioxide in the atmosphereare unlikely to come in contact with the positive electrode activematerial particles 41 in the active material layers 33 and 34 when thepositive electrode plate 531 is handled. Therefore, it is possible toreduce generation of lithium hydroxide and lithium carbonate on particlesurfaces of the positive electrode active material particles 41 in theactive material layers 33 and 34 due to contact with moisture and carbondioxide and change in a crystal structure on particle surfaces.Therefore, in the battery 500 using the positive electrode plate 531,compared to a battery using a positive electrode plate having noprotective conductive layers 535 and 536 on the active material layers33 and 34, the positive electrode plate 531 in which an IV resistance ofthe battery 500 is reduced can be obtained. In addition, since theconductive material 45 is included in the protective conductive layers535 and 536, compared to a positive electrode plate in which theconductive material 45 is not included in the protective conductivelayers 535 and 536, the conductivity of the positive electrode plate 531in the thickness direction GH can be improved. In addition, partssimilar to those of Embodiment 1 exhibit the same actions and effects asthose of Embodiment 1.

Examples and Comparative Examples

Next, results of tests performed to verify effects of the presentdisclosure will be described. The battery 500 of Embodiment 2 wasprepared as Example 1 and the battery 1 of Embodiment 1 was prepared asExample 2. In addition, as the comparative example, a battery includinga positive electrode plate (a positive electrode plate including onlythe positive electrode current collector foil 32 and the active materiallayers 33 and 34) having no protective conductive layer in the positiveelectrode plate 31 of Embodiment 1 was prepared. Parts other than thepositive electrode plate were the same as those of the battery 1 ofEmbodiment 1.

Then, IV resistances R of batteries of Examples 1 and 2 and thecomparative example were measured. Specifically, regarding batteries inwhich an SOC was adjusted to 50%, at an environmental temperature of 25°C., the batteries were discharged at a discharge current value I=5 C for5 seconds, and a battery voltage V1 when discharging was started and abattery voltage V2 after 5 seconds were measured. IV resistances R ofthe batteries were calculated according to R=(V1−V2)/I. In addition, anIV resistance value of the battery of the comparative example was set asa reference (=100%), and IV resistance ratios of Examples 1 and 2 wereobtained. The results are shown in FIG. 8.

The battery 500 of Example 1 had a lower IV resistance ratio (95%) thanthe battery of the comparative example, and the battery 1 of Example 2had a lower IV resistance (81%) than the battery 500 of Example 1. Thereason for this is inferred to be as follows. That is, in the battery ofthe comparative example, the positive electrode active materialparticles 41 included in the active material layers 33 and 34 of thepositive electrode plate came in contact with moisture in the atmosphereand reacted with water on particle surfaces of the positive electrodeactive material particles 41, and lithium hydroxide was generated(Li₂O+H₂O→2LiOH). In addition, the lithium hydroxide reacted with carbondioxide in the atmosphere and lithium carbonate was generated(2LiOH+CO₂→Li₂CO₃+H₂O). Lithium carbonate generated on particle surfacesof the positive electrode active material particles 41 was a resistor.In addition, when the positive electrode active material particles 41reacted with water and lithium ions were released from the positiveelectrode active material particles 41, a crystal structure of thepositive electrode active material particles 41 changed and insertionand removal of lithium ions in the positive electrode active materialparticles 41 became difficult. In the battery of the comparative exampleusing the positive electrode plate, an IV resistance R was thought to behigher for this reason.

On the other hand, in the batteries 1 and 500 of Examples 2 and 1, theprotective conductive layers 35, 36, 535, and 536 are provided on theactive material layers 33 and 34 of the positive electrode plates 31 and531, and thus moisture and carbon dioxide in the atmosphere wereunlikely to come in contact with the positive electrode active materialparticles 41 in the active material layers 33 and 34. Therefore, it ispossible to reduce generation of lithium hydroxide on particle surfacesof the positive electrode active material particles 41 in the activematerial layers 33 and 34, and additionally, generation of lithiumcarbonate due to contact with moisture and carbon dioxide and change ina crystal structure on particle surfaces. Therefore, in the batteries 1and 500 using the positive electrode plates 31 and 531, compared to abattery using a positive electrode plate having no protective conductivelayers 35, 36, 535, and 536 on the active material layers 33 and 34, itwas possible to reduce an IV resistance of the batteries 1 and 500. Thebatteries 1 and 500 of Examples 2 and 1 were thought to have a lower IVresistance ratio (lower IV resistance R) than the battery of thecomparative example for this reason.

In addition, in the battery 1 of Example 2, since the moisture absorbent47 was included in the protective conductive layers 35 and 36, even ifthe positive electrode plate 31 came in contact with moisture in theatmosphere, the moisture was absorbed by the moisture absorbent 47 inthe protective conductive layers 35 and 36. Therefore, it was possibleto reduce the amount of moisture that reached the active material layers33 and 34 below the protective conductive layers 35 and 36. Therefore,it was possible to effectively reduce generation of lithium hydroxide onparticle surfaces, and additionally, generation of lithium carbonate dueto moisture in contact with the positive electrode active materialparticles 41 in the active material layers 33 and 34, and change in acrystal structure on particle surfaces. The battery 1 of Example 2 usingthe positive electrode plate 31 was thought to have a lower IVresistance ratio (lower IV resistance R) than the battery 500 of Example1 for this reason.

While Embodiments 1 and 2 of the present disclosure have been describedabove, the present disclosure is not limited to Embodiments 1 and 2, andof course, it can be appropriately changed and applied without departingfrom the spirit and scope of the present disclosure. For example, inEmbodiments 1 and 2, in the one side second undried layer formingprocess S12 and the other side second undried layer forming process S15,the undried protective conductive layers 35 x and 36 x are formed byspray coating, but a coating method is not limited thereto. For example,the undried protective conductive layers 35 x and 36 x can be formed bydie coating and gravure coating.

What is claimed is:
 1. A positive electrode plate of a lithium ionsecondary battery comprising: a current collector foil; an activematerial layer including positive electrode active material particlescontaining lithium oxide on the current collector foil; and a protectiveconductive layer that does not include the positive electrode activematerial particles and includes a conductive material and a bindingagent on the active material layer.
 2. The positive electrode plate of alithium ion secondary battery according to claim 1, wherein theprotective conductive layer includes a moisture absorbent.
 3. Thepositive electrode plate of a lithium ion secondary battery according toclaim 2, wherein the moisture absorbent is a chemical moisture absorbentthat adsorbs water through a chemical reaction.
 4. The positiveelectrode plate of a lithium ion secondary battery according to claim 3,wherein the moisture absorbent is an anhydrite powder.
 5. The positiveelectrode plate of a lithium ion secondary battery according to claim 1,wherein the positive electrode active material particles included in theactive material layer have a property in which a pH of a liquiddispersion in which 1 g of the positive electrode active materialparticles is dispersed in 49 g of water is 11.3 or more.
 6. The positiveelectrode plate of a lithium ion secondary battery according to claim 1,wherein a layer thickness of the protective conductive layer is thinnerthan a layer thickness of the active material layer.
 7. The positiveelectrode plate of a lithium ion secondary battery according to claim 6,wherein the layer thickness of the protective conductive layer is 2 μmor more.
 8. A lithium ion secondary battery comprising: the positiveelectrode plate according to claim 1; and a negative electrode plate. 9.A method of producing a positive electrode plate of a lithium ionsecondary battery comprising: forming an undried active material layerincluding positive electrode active material particles containinglithium oxide on a current collector foil; forming an undried protectiveconductive layer that does not include the positive electrode activematerial particles and includes a conductive material and a bindingagent on the undried active material layer; and drying the undriedactive material layer and the undried protective conductive layersimultaneously and forming an active material layer and a protectiveconductive layer.